Electronic-type imaging devices of the type used, for example, in microscopes, telescopes, camcorders, thermal imagers, digital cameras, and especially with digital inspection systems (e.g., borescopes, and endoscopes), often are subjected to harsh physical operating conditions, and must embody a degree of survivability, in terms of mechanical durability, temperature tolerance, etc., which exceeds that required for similar electronics designed to operate in more benign environments, such as in clean rooms and climate-controlled office environments. In addition, these “hearty” imaging devices must be protected against environmental debris, such as dirt, dust, chemicals, and corrosive fluids, which might damage the optical elements (typically located in a tip of the imaging device) associated with the imaging devices introduced into hostile environments. “Optical elements” include well-known components used in imaging devices, such as miniature electronic image sensors (of the type, for example, used in CCD-type or CMOS-type imagers), mirrors, light sources (such as LEDs and laser diodes), and lenses.
In the past, windows, lenses, CHL assemblies, or other imaging portals leading to the optical elements of an imager (hereinafter, “imager window”) located in a tip of an imaging device have been afforded a modicum of protection by affixing a flat, optically transmissive element onto the imager window, such as a flat glass disk or plate. The flat element typically is bonded to the imager window using a UV adhesive, i.e., an adhesive cured under ultraviolet light during manufacturing. UV adhesives typically are used because of their generally quick cure times which, in turn, allow for a relatively short manufacturing step associated with the bonding.
This method of protecting the imager window, however, creates certain disadvantages with regard to the operation of the imaging device. For example, the use of a flat optical element bonded to an imager window, with UV adhesive interposed between the optical element and the imager window, gives rise to Newton's rings, a phenomenon observed when two materials with differing refractive indices are in close contact with each other. In practice, the presence of these rings (which typically are a series of concentric rings surrounding a central dark spot) reduces the image quality produced by the imaging device. Furthermore, the UV adhesives used to bond the flat optical element to the imager window, when stressed mechanically or thermally, tend to fail and crack, creating unwanted distortions and artifacts in the image produced by the imaging device. The failure of these adhesives also allows the flat optical element to disengage from the imager window, losing any protective benefit conferred by the presence of the flat optical element.
One prior art approach to addressing these problems has been to interpose bonding pads between the optical element and the imager window, and then to bond the optical element to the bonding pad in such a manner as to create a seal between the bonding pad and the optical element. That approach, however, is disadvantageous in that it requires additional materials and time-consuming alignment and bonding steps with regard to the bonding pads.
Another prior art approach has been to interpose adhesive, in the form of strips of preformed thermoplastic or thermosetting adhesives, between the optical element and the imager window at the edges where the optical element and imager window contact each other. However, this approach also is flawed in that the preformed adhesive strips cannot be tightly controlled when melted under heat, allowing the adhesive to “run” into the viewing area of the imager window.